(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a positive active material for a rechargeable lithium battery that can improve discharge capacity and cycle-life characteristics, a method of preparing the same, and a rechargeable lithium battery including the same.
(b) Description of the Related Art
In recent times, due to reductions in size and weight of portable electronic equipment, there has been a need to develop batteries for use in the portable electronic equipment where the batteries have both high performance and large capacity.
Batteries generate electric power by using materials capable of electrochemical reactions at positive and negative electrodes. For example, a rechargeable lithium battery generates electricity due to a change of chemical potential when lithium ions are intercalated/deintercalated at positive and negative electrodes.
The rechargeable lithium battery includes a material that can reversibly intercalate/deintercalate lithium ions as positive and negative active materials. It is fabricated by charging an organic electrolyte solution or a polymer electrolyte between the positive and negative electrodes.
In general, a positive active material of a rechargeable lithium battery includes a lithium composite metal compound. For example, LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (O<x<1), LiMnO2, and the like have been researched.
Manganese-based positive active materials such as LiMn2O4 or LiMnO2 are the easiest to synthesize, relatively thermally stable, less costly than the other materials, and environmentally friendly. However, these manganese-based materials have relatively low capacity.
LiCoO2 has good electric conductivity, high battery voltage, and excellent electrode characteristics. This compound is presently the most commercially available material and is made by Sony Corporation. However, it is relatively expensive and has low stability during charge-discharge at a high rate. LiNiO2 is currently the least costly of the positive active materials mentioned above and has a high discharge capacity, but it is difficult to synthesize and the least stable among the compounds.
In order to improve electrochemical characteristics of positive active materials for a rechargeable lithium battery, a composition control, a particle size, and a surface treatment for positive active materials have been continuously researched.
Japanese Patent laid-open No. 1997-55210 discloses a positive active material for a rechargeable lithium battery that is prepared by coating Li—Ni-based oxide with an alkoxide including Mn, Al, or Co and then heat treating.
U.S. Pat. No. 5,705,291 discloses a positive active material for a rechargeable lithium battery including a compound being capable of intercalating and deintercalating lithiums coated with an oxide of Al, B, or Si so as to minimize metal ion elution at high temperature.
However, the above suggested method cannot provide uniform coating when a core is a powder having a diameter of less than 10 μm, and an insulating coating layer is formed, which increases initial battery cell resistance.